1. Field of Invention
The present invention relates to a hydraulic servo-control of a servo-controlled gearbox.
2. Description of Related Art
Servo-controlled gearboxes have become increasingly widespread and are structurally similar to a traditional manual gearbox except in that the clutch pedal and the gear changing stick operated by the driver are replaced by corresponding electric or hydraulic servo-controls.
Using a servo-controlled manual gearbox, the driver only needs to send the order to shift to a higher gear or to a lower gear to a gearbox control unit and the gearbox control unit autonomously changes gear acting on both the engine and the servo-controls associated with clutch and gearbox.
Generally, the gearbox servo-control is of the hydraulic type and acts on a gearbox control shaft for transmitting both an axial displacement (i.e. along a central axis) for selecting the gear range, and a rotation about the central axis for engaging and disengaging the single gears, to the control shaft itself. Accordingly, the gearbox servo-control includes a linear hydraulic actuator mechanically coupled to the control shall for axially displacing the control shaft, and a rotary hydraulic actuator mechanically coupled to the control shaft for rotating the control shaft.
The gearbox servo-control further comprises a storing tank containing the control fluid used by the hydraulic actuators (typically oil) at room pressure, a hydraulic accumulator containing control fluid under pressure, a motor pump which draws the control fluid from, the storing tank and feeds the control fluid under pressure to the hydraulic accumulator, and a number of solenoid valves, which are adapted to selectively connect the chambers of the hydraulic actuators to the storing task and to the hydraulic accumulator.
Hydraulic accumulators of the known type comprise an outer housing which is internally divided into a first variable-volume chamber adapted to accommodate the control fluid and a second variable-volume chamber adapted to receive a gas under pressure (typically N2).
The division between the first and the second chambers is typically carried put by partition means arranged inside the outer housing, axially mobile inside the outer housing itself between an upper limit stop position and a lower limit stop position so as to vary the volumes of the first and second chamber, respectively, defined inside the outer housing. In particular, the partition means may comprise, for example, a bellows element made of a metal material, or a flexible partition membrane or even a piston.
In particular, hydraulic accumulators have become increasingly employed, which are provided with an outer housing shaped as a cup-shaped body, and with a piston arranged inside the outer housing, made axially sliding and mobile inside the outer housing itself.
While the above-described hydraulic accumulators are simple and inexpensive to be implemented, they have however the drawback that, if a traumatic and large damage occurs downstream of the hydraulic accumulator itself (e.g. a damage of about 10 mm in a piping, resulting in a considerable leakage of control fluid), a depression is generated such as to remove the partition means from the dedicated seat obtained into the outer housing and cause damage to the whole hydraulic servo-control.